Physical vapor deposition of piezoelectric films

ABSTRACT

A method of fabricating a piezoelectric layer includes depositing a piezoelectric material onto a substrate in a first crystallographic phase by physical vapor deposition while the substrate remains at a temperature below 400° C., and thermally annealing the substrate at a temperature above 500° C. to convert the piezoelectric material to a second crystallographic phase. The physical vapor deposition includes sputtering from a target in a plasma deposition chamber.

TECHNICAL FIELD

This invention relates to fabrication of piezoelectric devices, and moreparticularly to physical vapor deposition of piezoelectric films.

BACKGROUND

Piezoelectric materials have been used for several decades in a varietyof technologies, e.g., ink jet printing, medical ultrasound andgyroscopes. Conventionally, piezoelectric layers are fabricated byproducing a piezoelectric material in a bulk crystalline form and thenmachining the material to a desired thickness, or by using sol-geltechniques to deposit the layer. Lead zirconate titanate (PZT),typically of the form Pb[Zr_(x)Ti_(1-x)]O₃, is a commonly usedpiezoelectric material. Sputtering of PZT has been proposed.

More recently relaxor-lead titanate (relaxor-PT), such as(1-X)[Pb(Mg_(1/3)Nb_(2/3))O₃]—X[PbTiO₃] (PMN-PT),(1-X)[Pb(Y_(1/3)Nb_(2/3))O₃]—X[PbTiO₃] (PYN-PT),(1-X)[Pb(Zr_(1/3)Nb_(2/3))O₃]—X[PbTiO₃] (PZN-PT),(1-X)[Pb(In_(1/3)Nb_(2/3))O₃]—X[PbTiO₃] (PIN-PT), etc. have beenproposed as better piezoelectric materials. Relaxor-PT can offerimproved piezoelectric properties over the more commonly used PZTmaterial. However, large area thin film deposition of a relaxor-PT layerin a commercially viable manner has been not yet been demonstrated.

SUMMARY

In one aspect, a method of fabricating a piezoelectric layer includesdepositing a piezoelectric material onto a substrate in a firstcrystallographic phase by physical vapor deposition while the substrateremains at a temperature below 400° C., and thermally annealing thesubstrate at a temperature above 500° C. to convert the piezoelectricmaterial to a second crystallographic phase. The physical vapordeposition includes sputtering from a target in a plasma depositionchamber.

In another aspect, a physical vapor deposition system includes adeposition chamber, a support to hold a substrate in the depositionchamber, a target in the chamber formed of a piezoelectric material, apower supply configure to apply power to the target to generate a plasmain the chamber to sputter material from the target onto the substrate toform a piezoelectric layer on the substrate, and a controller configuredto cause the power supply to alternate between deposition phases inwhich the power supply applies power to the target and cooling phases inwhich power supply does not apply power to the target, with eachdeposition phase lasting at least 30 seconds and each cooling phaselasting at least 30 seconds.

Implementations may have, but are not limited to, one or more of thefollowing advantages.

A device that includes a layer of piezoelectric material can befabricated using physical vapor deposition in a commercially viableprocess. Overheating and damage to the target can be reduced, and thusthe danger of defects can be reduced and down-time of the processingchamber can be improved. PMNPT can be used for the piezoelectricmaterial, which can provide superior piezoelectric properties. A uniformcrystalline phase and stoichiometry can be achieved across the wafer.The process can also limit the presence of parasitic phases such as PbOxand pyrochlore, which can be detrimental to the piezoelectricproperties.

The details of one or more embodiments are set forth in the accompanyingdrawings and the description below. Other features, objects, andadvantages will be apparent from the description and drawings, and fromthe claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic cross-sectional view of a physical vapordeposition processing chamber.

FIG. 2 is a schematic cross-sectional illustration of a stack of layersin a piezoelectric device.

FIG. 3 is a schematic illustration of a timing diagram for applicationof power to a target in the physical vapor deposition processingchamber.

FIG. 4 is a schematic diagram illustrating phase of PMNPT as a functionof temperature and composition.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

Machining a piezoelectric layer from a bulk crystal and depositing apiezoelectric layer using sol-gel techniques are slow processes that arenot conducive to being performed in a semiconductor fabrication plant.Bulk crystals need to be machined in conventional machine shops. This isnot only expensive but also limits the ability of the piezoelectriclayer to be integrated into devices. Sol-gel processes require multiplerounds of deposition and curing, thus making the process time-consuming.Thus, deposition of a piezoelectric material by a physical vapordeposition process, e.g., sputtering, would be desirable.

Fabrication of thin films of piezoelectric material by physical vapordeposition (PVD) over large area semiconductor wafers, e.g., siliconwafers, has been challenging. For PVD of piezoelectric materials, e.g.,PZT or relaxor-PT, the target used in the sputtering process is aceramic material. However, the target used in the chamber can be subjectto cracking or other forms of damage. Moreover, even damage to thetarget that is not visible can result in the release of particulates,which can cause defects in the piezoelectric layer. being Without beinglimited to any particular theory, the ceramic materials used for thetarget have a poor thermal conductivity and, as a result, can accumulateso much heat (either directly due to from power applied by a powersource or radiatively from the substrate or both) that the target cracksor is otherwise damaged. Replacement of the target requires down-timefor the PVD system, which increases cost-of-ownership.

Hypothetically a cooling system could be used to keep the target at arelatively low temperature. However, in practice the additional coolingrequired is impractical. As noted above, the ceramic target has poorthermal conductivity and thus limited ability to transfer heat to acooling medium. While grooves or channels could be formed in the top ofthe target to increase thermal contact area and thus improve the thermaltransfer, such features can introduce non-uniformity in the electricfield and the deposition process.

A technique that may address these issues is to deposit thepiezoelectric layer while operating the PVD chamber in a manner thatkeeps the target at a lower temperature, e.g., to apply power at a lowerwattage and/or intermittently to permit the target to cool. In addition,the deposition process is followed by thermal annealing of thepiezoelectric layer so that the piezoelectric material achieves adesired crystalline structure.

FIG. 1 depicts a schematic representation of a chamber 100 of anintegrated processing system, e.g., an ENDURA system, suitable forpracticing the physical vapor deposition process discussed below. Theprocessing system can include multiple chambers, which can be adaptedfor PVD or CVD processes. For example, the processing system can includea cluster of interconnected process chambers, for example, a CVD chamberand a PVD chamber.

The chamber 100 includes chamber walls 101 that surround a vacuumchamber 102, a gas source 104, a pumping system 106 and a target powersource 108. Inside the vacuum chamber 102 is a target 110 and a pedestal112 to support the substrate 10. A shield can be placed inside thechamber to enclose a reaction zone. The pedestal can be verticallymovable, and a lift mechanism 116 can be coupled to the pedestal 112 toposition the pedestal 112 relative to the target 110. A heater orchiller 136, e.g., a resistive heater or a thermoelectric chiller, canbe embedded in the pedestal 112 to maintain the substrate 10 at adesired process temperature.

The target 110 is composed of the material to be deposited, e.g., leadmagnesium niobate-lead titanate for PMNPT, or lead zirconate titanatefor PZT. However, the target can have an excess of PbO_(x) relative tothe desired stoichiometry for the layer to be deposited to account forthe loss of lead due to its volatile nature. For example, the target canhave an excess of PbO of 1-20 mol %. The target itself should be ofhomogenous composition. he target 110 can be platinum (Pt) or Titanium(Ti) for deposition of other layers.

The gas source 104 can introduce an inert gas, e.g., argon (Ar) or xenon(Xe), or a mixture of an inert gas with a processing gas, e.g., oxygen,into the vacuum chamber 102. The chamber pressure is controlled by thepumping system 106. The target power source 108 may include a DC source,a radio frequency (RF) source, or a DC-pulsed source.

In operation, the substrate 10 is supported within the chamber 102 bythe pedestal 112, gas from the source 104 flows into the chamber 102,and the target power source 108 applies power to the target 110 at afrequency and voltage to generate a plasma in the chamber 102. Thetarget materials are sputtered from the target 110 by the plasma, anddeposited on the substrate 10.

If the target power source 108 is DC or DC-pulsed, then the target 110acts as a negatively biased cathode and the shield is a grounded anode.For example, a plasma is generated from the inert gas by applying a DCbias to the sputtering target 210 sufficient to generate a power densityof about 0.5 to 350 Watts per square inch, e.g., 100-38,000 W for a 13inch diameter target, and more typically about 100-10,000 W. If thetarget power source 108 is an RF source, then the shield is typicallygrounded and the voltage at the target 110 varies relative to the shieldat a radio frequency, typically 13.56 MHz. In this case, electrons inthe plasma accumulate at the target 110 to create a self-bias voltagethat negatively biases the target 110.

The chamber 100 may include additional components for improving thesputtering deposition process. For example, a power source 124 may becoupled to the pedestal 112 for biasing the substrate 10, in order tocontrol the deposition of the film on the substrate 10. The power source124 is typically an AC source having a frequency of, for example,between about 350 to about 450 kHz. When the bias is applied by thepower source 124, a negative DC offset is created (due to electronaccumulation) at the substrate 10 and the pedestal 112. The negativebias at the substrate 10 attracts sputtered target material that becomesionized. The target material is generally attracted to the substrate 10in a direction that is substantially orthogonal to the substrate 10. Assuch, the bias power source 124 improves the step coverage of depositedmaterial compared to an unbiased substrate 10.

The chamber 100 may also have a magnet 126 or magnetic sub-assemblypositioned behind the target 110 for creating a magnetic field proximateto the target 1210. In some implementations, the magnet rotates duringthe deposition process.

The operation of the chamber can be controlled by a controller 150,e.g., a dedicated microprocessor, e.g., an ASIC, or a conventionalcomputer system executing a computer program stored in a non-volatilecomputer readable medium. The controller 150 can include a centralprocessor unit (CPU) and memory containing the associated controlsoftware.

FIG. 2 illustrates a cross-section of a portion of a substrate 10 forfabrication of a device that includes a piezoelectric layer 16 formed ona semiconductor wafer 12. In particular, the substrate 10 includes oneor more layers 14 between the semiconductor wafer 12 and thepiezoelectric layer 16. The one or more layers 14 include at least afirst conductive layer 24 to provide a lower electrode. Depending on thematerial of the electrode and the piezoelectric material, the one ormore layers 14 can also include an adhesion layer 22 to improve adhesionof the conductive layer 24 to the semiconductor wafer 12, and a seedlayer 26 promote proper crystalline orientation of the piezoelectricmaterial in the piezoelectric layer 16. For some piezoelectricmaterials, the adhesion layer 22 and/or the seed layer 26 are notrequired and can be absent.

The semiconductor wafer can be a silicon wafer, or another semiconductorsuch as germanium (Ge). The silicon wafer can be a single crystalsilicon wafer, and can have a <001> crystallographic orientation,although other orientations can work.

Assuming the adhesion layer 22 and the seed layer 26 are present, thelayer stack 14 includes, in order, a silicon oxide (SiOx) layer 20, anadhesion layer 22, the lower conductive layer 24, and the seed layer 26.

The silicon oxide layer 20 can include SiO₂, SiO, or a combinationthereof. The silicon oxide layer 20 can be a thermal oxide, and can havea thickness of about 50-1000 nm. The silicon oxide layer 20 can be anamorphous layer.

The adhesion layer 22 can be a metal oxide layer. The stoichiometry ofthe metal oxide layer can MO₂, M₂O₃, or MO (with M representing themetal element), or another suitable stoichiometry of the metal andoxygen. In particular, the adhesion layer 22 can be formed of titaniumoxide, e.g., TiO₂, Ti₂O₃, TiO, or anther stoichiometry of titanium andoxygen. In some implementations, rather than a metal oxide layer, theadhesion is a pure metal or a metal alloy. Examples for the metal(either for the metal of the metal oxide, or for the pure metal orcomponent of the metal alloy) include titanium, chromium,chromium-nickel, and nickel. The adhesion layer 22 can be thinner thanthe silicon oxide layer 20. For example, a titanium oxide adhesion layer22 can have a thickness of 25-40 nm. The adhesion layer 22 can have acrystallographic orientation for facilitating a desired crystallographicorientation of the conductive layer 24. For example, a TiO₂ layer canhave a <001> orientation to facilitate a <111> orientation in a platinumconductive layer.

The first conductive layer 24 is formed of a conductive material such asplatinum, gold, iridium, molybdenum, SrRuO₃. The first conductive layer24 can be thicker than the adhesion layer 22, and can be thicker thanthe silicon oxide layer 20. For example, the first conductive layer 24can have a thickness of 50-300 nm. The first conductive layer 24 canhave a crystallographic orientation for facilitating a desiredcrystallographic orientation of the seed layer 26. For example, aplatinum layer can have a <111> crystallographic orientation tofacilitate a <001> orientation in a titanium oxide seed layer.

The seed layer 26 can be metal oxide. In particular, the seed layer 26can be an oxide of titanium or niobium. For example, the seed layer canbe TiO₂, Ti₂O₃, TiO, or another stoichiometry of titanium and oxygen.The seed layer 26 should have a uniform stoichiometry across the surfaceof the substrate 10. The seed layer 26 can have a crystallographicorientation for facilitating a desired crystallographic orientation ofthe piezoelectric layer 28. For example, a titanium oxide layer can havea <001> crystallographic orientation to facilitate a <001> orientationin a PMNPT piezoelectric layer. The seed layer 26 is thinner than theadhesion layer 22. For example, first seed layer 26 can be about 1-5 nm,thick, e.g., 2 nm.

The piezoelectric layer 16 is formed on the seed layer 26. Examples ofmaterial for the piezoelectric layer 16 include PZT and relaxor-PTmaterials. In particular, the material can be(1-x)[Pb(Mg_((1-y))Nb_(y))O₃]-x[PbTiO₃], where x is about 0.2 to 0.8,and y is about 0.8 to 0.2, e.g., about ⅔. Due to the presence of themetal oxide seed layer, the PMNPT material can be predominantly, e.g.,substantially entirely, a <001> crystallographic orientation. Thepiezoelectric layer can have a thickness of 50 nm to 10 microns.

A second conductive layer 30 is formed on the piezoelectric layer 16.The second conductive layer 30 can be the same material composition asthe first conductive layer 24, and can be the same thickness as thefirst conductive layer 24. For example, the second conductive layer 30can be platinum, and can have a thickness of 50-300 nm.

A voltage can be applied between the first and second conductive layers24, 30 in order to actuate the piezoelectric layer 16. Thus, the firstconductive layer provides 24 a lower electrode and the second conductivelayer 30 provides an upper electrode with the piezoelectric layer 16sandwiched therebetween.

To fabricate the layer stack 14, an oxide of SiO₂ can be grown on a Si<001> single crystal wafer by thermal processing in an oxygen-containingatmosphere. The thermal oxide can be grown to a thickness of 50-1000 nm,e.g., 100 nm. The thermal oxide can be formed on both sides of thesilicon wafer.

If the optional adhesion layer is included, then a metal layer whichwill provide the metal of the adhesion layer is deposited by PVD. Forexample, a titanium layer can be deposited. For example, the metal layercan be deposited with the substrate between room temperature and 600° C.and a power density of 0.5 to 20 Watts per square inch, e.g., about 1.5Watts per square inch, applied to the target. Deposition of the metallayer can be followed by annealing in a rapid thermal processing chamberor furnace in the presence of oxygen or air to form the adhesion layerin the form of the metal oxide layer, e.g., TiOx. The annealing can beat a temperature of 500-800° C., e.g., for 2-30 minutes. The resultingadhesion layer can have a thickness of 5-400 nm.

Next, the first conductive layer is deposited either on the adhesionlayer (if present), on the silicon oxide (if present), or directly onthe semiconductor wafer. For example, a platinum layer can be depositedat a substrate temperature of room temperature to 500° C., with a powerdensity of 0.5 to 20 Watts per square inch, e.g., 4-5 Watts per squareinch, applied to the target. Deposition of the first conductive layercan proceed until the layer has a thickness of 50-300 nm. The adhesionlayer, if present, provides improved adhesion between the platinum andthe silicon oxide.

If the optional seed layer is included, then a very thin metal layer,e.g., titanium, is deposited on the lower electrode, e.g., the platinumlayer, by a PVD (e.g., DC sputtering) or a CVD (e.g., ALD) technique. Inparticular, a titanium layer can be deposited, e.g., by DC sputtering.For example, the titanium seed layer can be deposited with the substrateat room temperature to 500 C and a power density of 0.5 to 4 Watts persquare inch, e.g., 1 Watt per square inch, applied to the target. Thethin metal layer can have a thickness of 1-5 nm. The thin metal layercan then be oxidized, e.g., heated in an oxidizing atmosphere to convertthe metal layer to a metal oxide, e.g., convert Ti to TiOx, to providethe seed layer. Additionally, the oxidized seed layer can also bedeposited directly by a PVD or CVD technique, e.g., TiOx deposition byRF sputtering or ALD.

Next, the piezoelectric layer is deposited, either on the seed layer ordirectly on the first conductive layer, by PVD. In particular,piezoelectric layer is deposited while keeping the target at arelatively low temperature, e.g., no higher than 175° C., e.g., nohigher than 150° C. For example, the target can be kept at roomtemperature to 150° C. A cooling system in the ceiling of the depositionsystem can be used to cool the target.

The deposition process can be run with the substrate at a relatively lowtemperature, e.g., no higher than 350° C., e.g., no higher than 300° C.For example, the cooling system in the pedestal can be operated to keepthe substrate at room temperature to 300° C. In contrast, a conventionaltemperature to deposit a relaxor-PT material is about 600-650° C. Whilesuch a high temperature can provide desirable crystalline properties inthe piezoelectric material upon deposition, the radiative heat from thesubstrate can be absorbed by the target, resulting in over-heating ofthe target.

Power applied to the target can be restricted to less than 1.5 W/cm²,e.g., less than 1.2 W/cm². For example, for a 13 inch diameter target,the power source can apply about 1000 W power (in contrast aconventional PVD operation would be conducted at 1.5 kW to 5 kW). Thislower power level results in less heat being generated in the target.

In addition, system can alternate between deposition phases in whichpower is applied to the target, and cooling phases in which no power isapplied and the target is permitted to cool down. For example, referringto FIG. 3, power is applied in pulses 200 during deposition phases 202,and power is not applied (or is applied at a significantly lower level)during cooling phases 204. The deposition phases 202 and cooling phases204 can last longer than 15 seconds, e.g., longer than 30 seconds, e.g.,longer than a minute. The deposition phases 202 and cooling phases 204can be up to about 10 minutes long. In some implementations, the coolingphases 204 can last longer than the deposition phases. For example, eachdeposition phase 202 can last three to five minutes, and each coolingphase 204 can last one to ten minutes, e.g., five to seven minutes. Thecooling phases allow the target to cool down between successivedeposition steps and can reduce or prevent cracking of the target.

After deposition of the piezoelectric layer, the substrate is removedfrom the PVD deposition chamber subjected to ex-situ thermal annealing,e.g. in a furnace or rapid thermal processing system. The substrate canbe heated to 500-850° C. In particular, for a piezoelectric layer formedof relaxor-PT material, the substrate can be heated to a temperatureabove the phase transition temperature between the perovskite andpyrochlore phases of the relaxor-PT material. As an example, FIG. 4illustrates the phase transition temperature as a function of thepercentage of PMN in the PMNPT (the percentage of PMN is given by X inthe chemical formula (1-X)[Pb(Mg_((1-Y))Nb_(Y))O₃]—X[PbTiO₃]). Forexample, for a piezoelectric layer that is about 70% PMN and 30% PT, thesubstrate should be raised to a temperature of about 750° C. or higher.

The temperature of substrate should be raised with sufficient speed tolimit formation of piezoelectric crystals in the pyrochlore phase, e.g.to below 50%. For example, the temperature can be raised from roomtemperature at a rate of 10-50° C. per second until the desiredtemperature is reached. Without being limited to any particular theory,the energy required for piezoelectric materials such as PMNPT totransition from the pyrochlore phase to the persovskite phase can begreater than the energy required to transition from the amorphous phaseto the persovskite phase. Thus, if temperature is raised slowly, thepiezoelectric material can enter and become “locked in” the pyrochlorephase. However, if the temperature is raised sufficiently rapidly, thepiezoelectric material does not have sufficient time to form crystals inthe pyrochlore phase.

The annealing can be conducted in a regular atmosphere, pure oxygenenvironment, pure nitrogen environment, a mixture of pure oxygen andnitrogen or in vacuum. Presence of oxygen during annealing can affectstoichiometry of the piezoelectric layer.

The annealing can significantly change crystalline grain size. Annealingcan also significantly increase the d33 coefficient, e.g., from 42 pm/Vto 197 pm/V.

A superior PMNPT layer can be formed with X approximately equal to ⅓ andY approximately equal to ⅔ in (1-X)[Pb(Mg_((1-Y))Nb_(Y))O₃]—X[PbTiO₃]).For example, X can be about 0.25 to 0.40, and Y can be 0.75 to 0.60.Such a composition can provide a favorable energy landscape at themorphotropic phase boundary (MPB) of PMNPT solid solution phase diagram.Due to the complex stoichiometry of PMNPT material, there is competitionbetween different phases to crystallize because of comparable energiesof formation. Thus, these ranges can be critical because slightdeviation can determine whether the crystal structure of PMNPT is cubic,orthorhombic, rhombohedral, tetragonal or monoclinic, which in turn canhave significant effect on piezoelectric properties.

Finally, a second conductive layer, e.g., a platinum film, is depositedby PVD on the piezoelectric layer. For example, the second platinum filmcan be deposited under the same conditions as the first platinum film.

A number of embodiments have been described. Nevertheless, it will beunderstood that various modifications may be made without departing fromthe spirit and scope of the disclosure. For example,

-   -   The system 100 illustrated in FIG. 1 is suitable for processing        a planar substrate 10, such as a semiconductor substrate, e.g.,        a silicon wafer, but the techniques discussed below could be        adapted to non-planar substrate.    -   The PVD process can use a self-ionized plasma (SIP). In the SIP        process, a plasma is initially ignited using an inert gas such        as argon. After plasma ignition, the inert gas flow is        terminated, and the deposition plasma is maintained by ions        generated from the sputtering target.    -   The upper electrode could be a different conductive material        than the lower electrode, e.g., a conductive material other than        platinum.    -   Although PMNPT and PZT are discussed, the technique of limiting        target temperature and thermally annealing the piezoelectric        layer can be applied to other piezoelectric compositions, e.g.,        PYN-PT, PZN-PT, PIN-PT, etc.

Accordingly, other embodiments are within the scope of the followingclaims.

1. A method of fabricating a piezoelectric layer, comprising: depositinga piezoelectric material onto a substrate in a first crystallographicphase by physical vapor deposition while the substrate remains at atemperature below 400° C., wherein the physical vapor depositionincludes sputtering from a target in a plasma deposition chamber; andthermally annealing the substrate at a temperature above 500° C. toconvert the piezoelectric material to a second crystallographic phase.2. The method of claim 1, wherein the second phase is a perovskitephase.
 3. The method of claim 2, wherein the first phase is an amorphousor a quasi-crystalline phase.
 4. The method of claim 1, wherein thepiezoelectric material is selected from a group consisting of leadmagnesium niobate-lead titanate (PMNPT), lead yttrium niobate-leadtitanate (PYNPT), lead zirconate titanate (PZT), lead zirconiumniobate-lead titanate (PZN-PT), and lead indium niobate-lead titanate(PIN-PT).
 5. The method of claim 4, wherein the piezoelectric materialis lead magnesium niobate-lead titanate (PMNPT).
 6. The method of claim5, wherein the PMNPT is (1-X)[Pb(Mg_((1-Y))Nb_(Y))O₃]—X[PbTiO₃], where Xis about 0.25 to 0.40 and Y is about 0.75 to 0.60.
 7. The method ofclaim 6, wherein thermally annealing the substrate comprises thermallyannealing the substrate at a temperature of above 500° C.
 8. The methodof claim 1, comprising depositing the piezoelectric material to athickness of 50 nm to 10 microns.
 9. The method of claim 1, comprisingmaintain the target at a temperature below 150° C. during the physicalvapor deposition of the piezoelectric material.
 10. The method of claim1, wherein the physical vapor deposition includes applying power to thetarget at a power less than 1.5 W/cm² of the target.
 11. The method ofclaim 1, wherein the physical vapor deposition includes alternatingbetween deposition phases in which power is applied to the target andcooling phases in which power is not applied to the target, eachdeposition phase lasting at least 30 seconds and each cooling phaselasting at 30 seconds.
 12. The method of claim 11, wherein the coolingphases are longer than the deposition phases.
 13. The method of claim11, wherein each deposition phase lasts at most five minutes.
 14. Themethod of claim 11, wherein each deposition phase lasts at most tenminutes.
 15. The method of claim 1, comprising maintaining a temperatureof the substrate below 400° C. during the physical vapor deposition. 16.The method of claim 1, comprising maintaining a temperature of thesubstrate by cooling a pedestal in the chamber that supports thesubstrate with a chiller.
 17. A physical vapor deposition system,comprising: a deposition chamber; a support to hold a substrate in thedeposition chamber; a target in the chamber formed of a piezoelectricmaterial; a power supply configure to apply power to the target togenerate a plasma in the chamber to sputter material from the targetonto the substrate to form a piezoelectric layer on the substrate; and acontroller configured to cause the power supply to alternate betweendeposition phases in which the power supply applies power to the targetand cooling phases in which power supply does not apply power to thetarget, each deposition phase lasting at least 30 seconds and eachcooling phase lasting at least 30 seconds.
 18. The method of claim 1,wherein the controller is configured to cause the power supply toapplying power during the deposition phases to the target at a powerless than 1.5 W/cm² of the target.
 19. The method of claim 17,comprising a chiller in the support, and wherein the controller isconfigured to operate the chiller to maintain a temperature of thesubstrate no higher than 400° C.
 20. A method of fabricating apiezoelectric layer, comprising: depositing a piezoelectric materialcomposed of lead magnesium niobate-lead titanate (PMNPT) onto asubstrate in an amorphous phase by physical vapor deposition while thesubstrate remains at a temperature below 400° C., wherein the physicalvapor deposition includes sputtering from a target in a plasmadeposition chamber; and thermally annealing the substrate at atemperature above 500° C. to convert the piezoelectric material to aperovskite phase, wherein the annealing includes raising the temperatureof the substrate at a rate sufficient that substantially no pyrochlorephase crystal is formed in the piezoelectric material.